The Fourth International Workshop on Chromosomes in Leukemia is designed to study the relationship of the karyotype in the leukemic cells of patients with acute nonlymphocytic leukemia (ANLL) with a number of clinical and biological features. The Workshop will summarize data obtained prospectively on patients studied since January 1, 1980. The study will continue through March 31, 1982. The data is being collected by 17 cooperating laboratories from the USA, Europe, Japan, Australia, and South Africa. Complete clinical and cytogenetic data will be entered in the computer registry at The University of Chicago. Karyotypes of each of the normal and abnormal clones will be reviewed by Workshop participants. Slides of the bone marrow will be classified according to the FAB classification. The Workshop participants will determine whether the chromosome pattern remains a significant prognostic indicator as treatment of ANLL becomes more effective. The relationship of occupation and hobbies will be correlated with the karyotype of the leukemic cells. The methods for using mutant leukemic cells for gene mapping will be reviewed.